Introduction to Calibration Boards

Introduction to Calibration Boards

In machine vision applications, camera calibration technology requires accurate internal and external camera parameters as inputs and prerequisites for reconstruction algorithms. The most popular camera calibration algorithm is the one proposed by Tsai in 1987 [Tsai1987]. Tsai's method uses a three-dimensional calibration object with non-coplanar special calibration markers to provide the correspondence between image points and their corresponding three-dimensional space points, allowing for the calculation of calibration parameters.

Calibration algorithms enable the calculation of the camera's projection matrix and provide three-dimensional measurement information for the scene. Without absolute translation, rotation, or scaling parameters in the real-world scene, calibration can achieve a level of metric reconstruction similar to transformations. To meet the needs of high-precision visual calibration applications in industry, a series of machine vision calibration board products have been introduced. These products allow machine vision software to easily obtain camera calibration result data and are used as specialized high-precision calibration tools for universities, research institutions, and machine vision integrators.

• • Material One: Soda Glass

    • In Russia, calibration boards are silver in color; in Germany, Austria, and neighboring countries, they're blue; in the United States and Japan, they're yellow. The color is determined by the coating process, so it can't be used to judge the quality of the calibration board. High-end domestic calibration boards are typically yellow, while other colors have relatively lower costs. Standard production processes include polishing, cleaning, and post-coating. A solid coating with minimal defects is a sign of good quality.
    • If any step in the manufacturing process is reduced or if low-quality machinery is used, the resulting product can be much cheaper, but the quality will be much lower. For instance, a 63mm x 63mm calibration board, if produced following the standard process, costs at least a hundred yuan when produced in bulk.

  • Material Two: Ceramic Base

    • Ceramic-based calibration boards have a low thermal expansion coefficient, high strength, high hardness, good abrasion resistance, low thermal conductivity, and strong acid and alkali resistance. Their surface is well-treated for diffuse reflection, which resolves the reflection problem in glass-based calibration boards when using front lighting. This allows for better recognition of the calibration board pattern's detail, enabling higher calibration and measurement accuracy.

  • Material Three: Quartz

    • Quartz-based calibration boards have excellent light transmittance and a low expansion coefficient. They are less affected by environmental temperature during calibration processes. However, quartz is expensive. There are two types of quartz materials: JGS1, which is more costly, and JGS2, which is cheaper but contains bubbles. When selecting a calibration board, consider its intended environment, required accuracy, and other factors. Differences in accuracy and visible flaws can lead to significant differences in the product's batch price.

  Types of Calibration Boards

  Calibration boards generally come in two types:

  1. Solid Circle Array Pattern
     This pattern consists of a series of solid circles arranged in a grid-like structure.

  2. Checkerboard Pattern
     This pattern resembles a chessboard, with alternating light and dark squares arranged in a grid.

  Both types are used in machine vision applications for camera calibration and other optical measurement tasks. The choice between them depends on the specific application requirements and the desired calibration precision.